In a case where a slip restraining control for restricting an occurrence of a slip at a wheel, such as a so-called anti-skid control (ABS control), a so-called traction control (TCS control) and the like is executed while a vehicle travels on a road surface having different friction coefficients in a lateral direction corresponding to a right-left direction of the vehicle (hereinafter referred to as a μ split road surface and a control for restricting the occurrence of the slip at the wheel while the vehicle travels the μ split road surface is referred to as a μ split control), a difference is generated between a longitudinal force of right wheels (hereinafter referred to as a right-wheel longitudinal force) and a longitudinal force of left wheels (hereinafter referred to as a left-wheel longitudinal force). The longitudinal force is a frictional force in an acceleration-deceleration direction generated between the road surface and a tire. Additionally, the longitudinal force is referred to also as a braking/driving force. A yaw moment for deflecting the vehicle is generated due to the difference between the right-wheel longitudinal force and the left-right longitudinal force (hereinafter referred to as a longitudinal force difference attributing yaw moment).
In order to prevent the deflection of the vehicle, occurring due to the longitudinal force difference attributing yaw moment, a yaw moment needs to be generated in a direction where the longitudinal force difference attributing yaw moment is reduced (offset) by adjusting the steering angle of the front-wheels to be directed opposite from a vehicle deflecting direction. An adjusting operation of the front-wheel steering angle to be directed in a direction opposite from the vehicle deflecting direction is referred to as a counter-steering operation.
A steering control device disclosed in JP2005-112285A automatically executes the counter-steering operation without involving a steering wheel operation by a driver (hereinafter referred to also as a counter-steering control). More specifically, a reference steering angle of the front wheels is determined on the basis of a rotational angle of a steering wheel. A first adjustment steering angle of the front wheels directed so as to restrain the vehicle deflection, occurring due to the difference between the right-wheel longitudinal direction and the left-wheel longitudinal direction, is determined. Then, a second adjustment steering angle of the front wheels, directed so as to approximate an actual value to a target value (i.e. directed so as to approximate a deviation in motion state quantity to zero), is determined on the basis of a deviation between the actual value and the target value of the motion state quantity in a yaw direction of the vehicle (i.e. deviation in the motion state quantity, such as a deviation between a target yaw rate and an actual yaw rate, and the like). A target steering angle of the front-wheels is determined by adding the first and the second adjustment steering angles to the reference steering angle. Then, the front-wheels are controlled by means of an actuator so that the actual steering angle of the front-wheels corresponds to the target steering angle in the steering angle control device disclosed in JP2005-112285A.
In the counter-steering control executed in the steering control device disclosed in JP2005-112285A, the reference steering angle is adjusted by using not only a feed-forward control utilizing the first adjustment steering angle, determined on the basis of the longitudinal force difference between the right and left wheels, but also a feedback control utilizing the second adjustment steering angle, determined on the basis of the deviation in the motion state quantity, for the following reasons. If the vehicle deflection, occurring due to the longitudinal force difference, is accurately prevented only by the feed-forward control utilizing the first adjustment steering angle, the feedback control utilizing the second adjustment steering angle is not needed. However, an error of detecting the longitudinal force difference used for calculating the first adjustment steering angle, fluctuation in changing characteristics of the yaw moment relative to changes of the front-wheel steering angle and the like unavoidably occur, which results in generating an error in the feed-forward control utilizing the first adjustment steering angle. The error in the feed-forward control emerges as the deviation in the motion state quantity. Therefore, in order to compensate the error in the feed-forward control utilizing the first adjustment steering angle, the feedback control utilizing the second adjustment steering angle, determined on the basis of the deviation in the motion state quantity, is also executed.
FIG. 19 illustrates a case where the μ split control is started when a brake operation is performed while the vehicle is driven on the μ split road surface, which curves towards the left in FIG. 19 and where the friction coefficient of road surface contacting turning inner wheels is low (low μ) and the friction coefficient of the road surface contacting turning outer wheels is high (high μ). In this case, the difference in the longitudinal forces occurs, thereby generating the longitudinal force difference attributing yaw moment in a turning outer direction (a clockwise direction when the vehicle is viewed from above), as illustrated in FIG. 19.
When the counter-steering control is executed while the vehicle is in the above-described state, the front-wheel steering angle is adjusted to a direction by which the front-wheel steering angle is increased towards the turning inner side by the first and the second adjustment steering angles. In other words, the counter-steering operation is automatically executed in the above-described state. As a result, the deflection of the vehicle, occurring due to the longitudinal force difference, is appropriately restricted.
On the other hand, FIG. 20 illustrates a case where the brake operation is performed while the vehicle travels on a road surface curving towards the left in FIG. 20 and having a constant friction coefficient, specifically, a constant low μ of the friction coefficient. In this case, the longitudinal force difference attributing yaw moment is not generated because the difference in the longitudinal forces does not occur. However, understeer may occur in this case. An understeer tendency emerges as the deviation in the motion state quantity, i.e. the target yaw rate becomes larger than the actual yaw rate.
In a case where the counter-steering control is executed in the state where the understeer occurs at the vehicle, the first adjustment steering angle based on the longitudinal force difference is calculated as zero, or a value approximating to zero, because the longitudinal force difference does not occur. On the other hand, the second adjustment steering angle based on the deviation in the motion state quantity is determined to be a value by which the front-wheel steering angle is increased towards the turning inner side (i.e. towards the left in FIG. 20) in order to approximate the deviation in the motion state quantity to zero, for example, by increasing the actual yaw rate. As a result, the front-wheel steering angle is adjusted to the direction by which the front-wheel steering angle is increased towards the turning inner side (i.e. towards the left in FIG. 20) by the second adjustment steering angle.
In the case where the understeer occurs at the vehicle, a lateral force (a cornering force), which may be generated at the front-wheels, has already saturated, therefore, even if the front-wheel steering angle is increased towards the turning inner side, the lateral force is not increased. Hence, in the situation where the understeer tends to occur, i.e. the situation where a side-slip tends to occur at the wheel, it may be preferable that the front-wheel steering angle is prevented from being adjusted by the second adjustment steering angle to the direction by which the front-wheel steering angle is increased towards the turning inner side.
Described above is the case where an adjustment control of the front-wheel steering angle is executed as the counter-steering control. The above-described phenomena also occur in a case where the adjustment control is executed on a rear-wheel steering angle as the counter-steering control. The case where the adjustment control is executed on the rear-wheel steering angle as the counter-steering control will be described in detail in accordance with FIGS. 21 and 22. FIGS. 21 and 22 illustrate a case where the rear-wheel steering angle (reference angle) is controlled in the same direction as the front-wheel steering angle is controlled, in response to the front-wheel steering angle.
FIG. 21 illustrates a case where the μ split control is started by performing the brake operation while the vehicle travels the μ split road surface turning towards the left in FIG. 21 and having a high friction coefficient (high μ) at the turning inner side and a low friction coefficient (low μ) at the turning outer side. In this case, the difference in the longitudinal forces is generated, thereby generating the longitudinal force difference attributing yaw moment in a direction towards the turning inner side (i.e. in a counterclockwise direction when the vehicle is viewed from above), as illustrated in FIG. 21.
If the same counter-steering control as the one executed to adjust the front-wheel steering wheel is executed on the rear-wheel steering angle in order to generate a yaw moment for reducing (offsetting) the longitudinal force difference attributing yaw moment, the rear-wheel steering angle is adjusted to a direction by which the rear-wheel steering angle is increased towards the turning inner side by the first and the second adjustment steering angles. As a result, the vehicle deflection occurring due to the difference in the longitudinal forces is appropriately prevented.
On the other hand, FIG. 22 illustrates a case where the brake operation is performed while the vehicle travels on a road surface curving towards the left in FIG. 22 and having a constant friction coefficient, specifically, a constant low μ of the friction coefficient. In this case, the longitudinal force difference attributing yaw moment is not generated because the difference in the longitudinal forces does not occur. However, oversteer may occur in this case. An oversteer tendency emerges as the deviation in the motion state quantity, i.e. the actual yaw rate becomes larger than the target yaw rate.
In the case where the counter-steering control is executed on the rear-wheel steering angle in the state where the oversteer occurs at the vehicle, the first adjustment steering angle based on the longitudinal force difference is calculated as zero, or the value approximating to zero, because the longitudinal force difference does not occur. On the other hand, the second adjustment steering angle based on the deviation in the motion state quantity is determined to be the value by which the rear-wheel steering angle is increased towards the turning inner side (i.e. towards the left in FIG. 20) in order to approximate the deviation in the motion state quantity to zero, for example, by decreasing the actual yaw rate. As a result, the rear-wheel steering angle is adjusted to the direction by which the rear-wheel steering angle is increased towards the turning inner side (i.e. towards the left in FIG. 20) by the second adjustment steering angle.
In the case where the oversteer occurs at the vehicle, the lateral force (the cornering force), which may be generated at the rear-wheels, has already saturated, therefore, even if the rear-wheel steering angle is increased towards the turning inner side, the lateral force is not increased. Hence, in the case where the adjustment control is executed on the rear-wheel steering angle as the counter-steering control, if the vehicle is in a state where the oversteer tends to occur (i.e., a state where the side-slip tends to occur at the wheel), it may be preferable that the rear-wheel steering angle is prevented from being adjusted by the second adjustment steering angle to the direction by which the rear-wheel steering angle is increased towards the turning inner side.
A need thus exists to provide a steering control device for a vehicle executing at least a counter-steering control of adjusting a steering angle of a wheel by an adjustment steering wheel on the basis of a motion state quantity (a motion state quantity deviation) in a yaw direction, that appropriately determines the adjustment steering angle in a state where a side-slip of the wheel tends to occur.